Normal baking in cycles is painfully slow
The file you provided uses 13GB of RAM at its peak when baking, which is obviously more than  your entire system has. It is taking a long time because your system is hitting the swap HDD/SSD hard. For comparison, a Ryzen 7 with enough ram can bake the scene in 4 minutes.

You can try to lower the multires level on the highres mesh and see if that helps lower the memory usage.

Alternatively, you can use Blender Render's Bake from Multires function to directly bake the normal map.


Transfer the UV map from the low-poly model to the high-poly model, by select the target(hipoly), then the source(lowpoly), then Ctrl+L, and select Transfer UV.
Assign an empty image to the high-poly model
Switch to Blender Render and select the Hi-poly model (we don't need the low poly model)
Bake with 'Bake from Multires' turned on.




This is actually even faster than using Cycles baking, and much less setup.